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Re: [tree-ssa/java] remaining null-pointer-check hits


In message <200405042035.29902.stevenb@suse.de>, Steven Bosscher writes:
 >> I've also made a trivial extension to DOM which allows it to propagate the
 >> non-null property through PHI nodes.  ie, if all arguments to a PHI node
 >> are known to be nonzero, then the result of the PHI node must also be
 >> nonzero.  That fixed the half-dozen cases we missed in GCC itself.
 >
 >Isn't this something ccp can do instead?  Or did you add this to keep
 >all the null checks together in one file?  It's a bit unintiutive that
 >DOM would have to look through dominance frontiers ;-)
CCP could do it as well.  CCP could in fact do just about any propagation 
that DOM currently does.  In fact const/copy propagation CCP is just a more
aggressive version of what is currently done by DOM.

It's _easier_ to do it in DOM as we've already got all the little bits
which tell us that a particular statement used a particular SSA_NAME in
a way which guarantees that the SSA_NAME has a nonzero value.

 >> The net result is we're left one case we're not handling.  Specifically if
 >> we have multiple paths where a pointer is dereferenced and there is no
 >> PHI node at their join point.  ie
 >>
 >>     BLOCK1      BLOCK 2
 >>    p1->field   p1->field
 >>        \          /
 >>         \        /
 >>          \      /
 >>           \    /
 >>           BLOCK 3
 >>           if (p1) ...
 >
 >How many of these appear in other important software (e.g. kernel, glibc, 
 >SPEC)?
Unknown at this time.  However, I would expect that number to be quite
small.  It'll be easy enough to check.  And again, if it turns out there's
significant numbers of these still lying around it's simply a matter of
plugging in the right code into a global propagation framework.

jeff



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